At some point between Ubuntu 7.04 and 8.10, it seems that the file-roller "Extract here" context-menu item in Nautilus (Ubuntu's file explorer) began opening a new window for the folder that had just been extracted.

I personally find this exceptionally annoying, and have finally looked up how to disable it. It only takes a few simple steps; I have only verified these in 8.04 LTS (Hardy Heron), though.

Open gconf-editor (Applications > System Tools > Configuration Editor); you can also run it from the terminal as gconf-editor.

Navigate to the /apps/file-roller/dialogs/extract/recreate_folders section.